1. Field of the Invention
The present invention relates to speaker diaphragms used for audio speakers, manufacturing methods of the same, and dynamic speakers.
2. Description of the Related Art
Conventional diaphragms for dynamic speakers are in most cases made of paper pulp, plastic or metal such as aluminum. These conventional diaphragms are made by press molding of a homogeneous material, and therefore, velocities of sounds propagated from the center of the diaphragm in various radial directions thereof are equal to each other. Accordingly, with regard to the conventional diaphragms, standing waves are always generated and resonance points appear. Moreover, sound pressure frequency characteristics are deteriorated due to split vibrations in low frequency regions. Furthermore, feeling of being at a live performance or the like will be lost.
In addition, although the diaphragms made of paper pulp or plastic have relatively high internal loss coefficients, they are not good in terms of stiffness (Young's modulus) and sound wave velocities thereof are low. Definition of the reproduced sounds is therefore not satisfying. In contrast, the diaphragms made of metal such as aluminum have high sound wave velocities. However, the internal loss coefficients thereof are low, and therefore, amplitude attenuation is small. Consequently, there is a drawback that reproduced sounds are not clearly divided (that is, sounds are not sharp).
Instead of such diaphragms made of existing materials, which have both advantages and drawbacks, wooden diaphragms are desired to be realized as speaker diaphragms that can reproduce more natural sounds.
The sound wave velocities of wood vary according to the kinds of wood. However, in general, wood has higher sound wave velocities than paper pulp. Moreover, the sound wave velocities in the grain direction and those in a direction perpendicular to the grain are different from each other. Wood has an anisotropic propagation characteristic, in which the propagation velocities in the grain direction (referred to as a “longitudinal direction”) are relatively high, while the propagation velocities in the direction perpendicular to the grain (referred to as a “horizontal direction”) are relatively low. Moreover, in general, wood has internal loss coefficients appropriate as speaker diaphragms, and is light and strong (high stiffness).
Hence, with regard to the wooden diaphragms, in which wood is used as basis material as it is, standing waves are not generated normally and resonance points hardly appear. Therefore, it can be expected that sound pressure frequency characteristics in low frequency regions are improved, and definition and sharpness of the reproduced sounds can be enhanced compared to the existing diaphragms made of paper pulp.
Various manufacturing methods have already been proposed in, for example, the following publicly known documents as manufacturing methods of wooden diaphragms or dynamic speakers employing the wooden diaphragms.
In Japanese Patent Laid-Open No. Sho 62(1987)-224196 (Patent Document 1), described is a speaker using, as a flat diaphragm for the speaker, a thin, flat wooden board treated to a wood plastic combination (WPC). Here, the thin, flat wooden board is impregnated with plastic such as polyester.
In Japanese Patent Laid-Open No. Hei 1(1989)-288100 (Patent Document 2), described is a cone diaphragm 5 manufactured through a manufacturing procedure shown in FIGS. 1A and 1B. As shown in FIG. 1A, a wooden sheet 1, a development of an approximately horn shape, which has a circular cutout portion 1a in the center thereof and a sector-shaped cutout portion 1b extended from the cutout portion 1a, is removed from one sheet of thin wooden board 4. Then, as shown in FIG. 1B, both edge portions b1 and b2 of the sector-shaped cutout portion 1b are overlapped with each other on an overlapping portion 1c, and adhered with an adhesive to be formed into the approximately horn shape. Thereafter, the wooden sheet 1 is press molded to be a cone diaphragm 5.
In addition, in Patent Document 2, it is described that a plurality of small wooden pieces 2a to 2L shown in FIG. 2A are adhered with an adhesive, made into an approximately horn shape as shown in FIG. 2B, and then press molded to be a cone diaphragm 2. It is also described that physical moisture-proof treatment or chemical moisture-proof treatment is performed onto the cone diaphragms 2 and 5.
In Japanese Patent Laid-Open No. Hei 5(1993)-83792 (Patent Document 3), an acoustic diaphragm molded in a circular cone shape is described. This diaphragm is produced as follows: a composite sheet made up of a sliced, extremely thin wooden sheet with a thickness of 80 ″m or less and a non-woven cloth of adhesive resin attached on the back surface thereof are plasticized with a plasticizer; and a plurality of the composite sheets are stacked, heated and pressed.
In Japanese Patent Laid-Open No. Hei 6 (1994)-178386 (Patent Document 4), described is a cone diaphragm which is produced as follows: a thin, flat wooden board is cut into a sector shape; the thin wooden board is boiled in hot steam, hot water, or an alkaline solution of pH 10 or more in order to prevent cracks upon molding; and the thin wooden board is press molded into a circular cone shape.
In Japanese Patent Laid-Open No. Hei 10(1998)-304492 (Patent Document 5), described is a speaker diaphragm which is produced as follows: a thin non-woven cloth or Japanese paper is adhered on one surface of a piece of wooden sheet to make an adhered sheet; the adhered sheet is impregnated with a lubricant to get tension and flexibility; and the adhered sheet impregnated with the lubricant is hot press molded to be made into an approximately horn shape.
In Japanese Patent Laid-Open No. 2000-59883 (Patent Document 6), described is a speaker diaphragm which is produced as follows. In order to produce a wooden speaker diaphragm in which no wrinkles or cracks occur upon press molding and deterioration with time is small, on one surface of a piece of wooden sheet, a thin non-woven cloth, Japanese paper or a carbon sheet is adhered to make an adhered sheet, and the adhered sheet is impregnated with thermosetting resin. Then, the adhered sheet is hot press molded to be made into an approximately horn shape.
With respect to a wooden speaker diaphragm, it is a key to deal with the characteristics of wood, which is a basis material that easily cracks, the poor moldability thereof, and the deterioration thereof with age.
In above-mentioned Patent Document 1, a manufacturing method of molding and making the diaphragm into an approximately horn shape (or cone shape) is not described, since the diaphragm is a flat diaphragm. In the manufacturing methods of Patent Document 2 shown in FIGS. 1A and 1B and FIGS. 2A and 2B, respectively, the wooden sheet 1 and the small wooden pieces 2a to 2L are molded while the characteristics of the wood are maintained as they are. Therefore, cracks may occur upon handling or press molding of the wooden sheets. Moreover, since the wooden sheets are adhered to each other with an adhesive, there are problems that desired sound characteristics cannot be obtained and accuracy in dimensions cannot be secured because the adhered portions become obstacles.
As for the method described in Patent Document 3, in which a plurality of extremely thin wooden sheets are stacked and press molded, accurate processing thereof is rather difficult. Moreover, since there are more adhesive layers, the weight of the diaphragm itself becomes heavier due to the weight of the adhesive. As a consequent, desired sound characteristics cannot be obtained. The great amount of adhesive imposes adverse effects on the sound pressure frequency characteristics. Therefore, the method is not preferable.
In the manufacturing method described in Patent Document 4, a boiling process using hot steam or an alkaline solution of pH 10 or more is included. This method removes even lignin, a constituent material in the wood. As a result, characteristics intrinsic to the wood cannot be exhibited, which imposes adverse effects on the sound quality.
Above all, in the manufacturing methods described in Patent Documents 2 to 4, defects such as cracks, chaps and wrinkles are often generated. Therefore, in considering the mass production of the diaphragms, yield of thereof is extremely poor. Thus commercialization thereof have not been realized.
The manufacturing methods described in Patent Documents 5 and 6 are more suitable than those in Patent Documents 2 to 4 in terms of mass production. However, it cannot be said that a manufacturing method which offers sufficient moldability and excellent mass productivity has been accomplished. Accordingly, further improvement in moldability, realization of beautifully finished wooden diaphragms, and establishment of mass productivity are desired in order to commercialize the wooden diaphragms.